[r3bl_core] create decl macro to make it easy to generate global mutable variables (singleton)

[nazmulidris]

create_global_singleton! decl macro

• This macro was created for tls crate, in rust-scratch monorepo and moved here.

• Once this issue is closed and its branch is rebased into main, tls repo in rust-scratch monorepo should use the macro from here.

• The tls/Cargo.toml has already been changed to get the r3bl_core dependency from it's main branch.

• Replace all ignored doc tests with either: no_run (just compile) or compile & run.